Information on size and velocity of spherical objects including particles, droplets, bubbles, etc., is important for a wide range of applications. These applications include, for example, fuel spray combustion analysis and control for the automotive industry, aircraft gas turbine combustion, inhaler manufacturing for the pharmaceutical industry, household spray systems manufacturing, agricultural pesticide application, aircraft icing analysis and control, spray nozzle manufacturing, atmospheric aerosol analysis, atmospheric studies, and various combustion related applications.
Typically, various laser light scattering interferometry techniques are used to determine the size and velocity of spherical objects, such as particles, drops, bubbles, etc. According to these techniques, spherical objects pass the intersection point of two crossed laser beams generated from the same laser. The two crossed laser beams form a sample volume at the intersection point. The light scattered by the spherical object, as it passes through the sample volume, produces an interference fringe pattern at the plan of the detector. The spatial period of the interference fringe pattern produced by the light scattered from the spherical object, as it passes through the sample volume, may be used to determine the size of the spherical object and a velocity component of the spherical object. The laser light scattering interferometry techniques may include a laser Doppler velocimetry (“LDV”), a laser Doppler anemometry (“LDA”), a phase Doppler interferometry (“PDI”), phase Doppler particle analyzer technique (“PDPA”), phase Doppler anemometer technique (“PDA”).
The measurements of the size and velocity of the spherical objects involve many parameters that may change over the measurement. For example, temporal and spatial characteristics of the flow of the spherical objects may change, e.g., due to changes in droplet size distribution, and due to gradients of the flow velocity. In many applications, e.g., for gas turbine and automotive fuel sprays, and industrial spray studies, the number density and velocity of particles changes dramatically from location to location. The small particle size and random attenuation of the scattered light caused by other droplets that pass the laser beams close to the sample volume may produce low signal to noise ratio.
Generally, the phase shifts of the signals that are produced by photodetectors are measured to estimate the spatial period of the interference fringe pattern produced by the light scattered from the spherical objects. The phase shifts may vary with measurement conditions, e.g., signal frequency, gain of the photodetector, and other instrument parameters.
These parameters, e.g., varying temporal and spatial characteristics, varying number density of the particles, varying particle speed, low signal-to-noise ratio, varying phase shifts, increase measurement uncertainty, cause measurement errors, and impact the measurement accuracy and reliability. Therefore, the measurements of the size and velocity of the spherical objects require frequent attention to the measurement conditions which may be time-consuming and improper setup may leave the instrument prone to errors.